OOS 24-3: Spotted knapweed and the mystery of the missing weapons of mass destruction

Background/Question/Methods The novel weapons hypothesis posits that exotic plants dominate ecosystem through their novel chemistry, particularly alleopathic interference with naïve native competitors. Much of the evidence for the role of allelopathy as a potent mechanism driving invasion comes from spotted knapweed (Centaurea stoebe micranthos, also known as Centaurea maculosa). Competition experiments using activated carbon originally suggested allelopathic activity. Further research demonstrated that spotted knapweed exudes the compound catechin from its roots, and that this compound can have toxic effects on naïve North American plants. Additionally, high concentrations (over 1,000 ppm, with a high over 7,000 ppm) of catechin were reported in soil samples from knapweed sites. These exciting findings led us to initiate research on this system. However, our results and others challenge the role of catechin in spotted knapweed invasions. Results/Conclusions In our research, we found that production by individual plants was 3 orders of magnitude less than previously reported, and documented only 0.11 ppm in soil samples. Sampling by the research group that did the original work revealed that some negative controls (i.e. blanks) for soil samples were contaminated with catechin, causing them to call into question their earlier work in which large amounts were reported from soils. Further, we demonstrated that the compound is quite unstable in water and wet soil, suggesting that even if pulses of it are released into the soil, it would break down following precipitation. We found variable toxicity of the compound among samples from varying sources, and further analysis revealed that toxic samples were comprised of up to 30% 2,4-D (a herbicide) and 30% 8-hydroxyquinolin (a known anti-microbial agent). If contamination by herbicide was widespread, it could explain previous reports of high toxicity. New research done by another laboratory has revealed that the presence of endophytes might explain the initial results seen in activated carbon/competition experiments. Indeed, it appears that the soil microbial community may play a vital role in both suppressing and facilitating invasion by exotic plants. While novel weapons may facilitate some invasions, catechin does not appear to drive the invasion of spotted knapweed. Other compounds and other processes should be studied to better understand the ability of this plant to dominate communities.